A new, noninvasive, and low-cost method for the early detection and
monitoring of osteoarthritis (arthritis caused by wear and tear) may be
on its way, thanks to research by UC Santa Barbara scientists from the
Department of Chemical Engineering and the Department of Materials.

By studying patterns of friction between cartilage pads, the
researchers discovered a different type of friction that is more likely
to cause wear and damage. Their work suggests ways to detect this
friction, and points to new research directions for getting to the root
cause of arthritis. The findings are published in the recent issue of
the Proceedings of the National Academy of Science.

Imagine going to the doctor for your aching knees. For some, this may
involve uncomfortable needle sticks to draw blood for lab tests, or the
extraction of the fluid surrounding the aching joint. But what if your
doctor could actually listen to your body, monitoring the way
your knees sound as they bend and flex? Research by Jacob Israelachvili,
UCSB professor of chemical engineering and materials; grad
student researcher Dong Woog Lee; and postdoctoral researcher Xavier
Banquy says that it's possible.

For their work, they
used an instrument called a Surface Forces Apparatus (SFA), a device
that measures the adhesion and friction forces between surfaces, in this
case cartilage –– the pad of tissue that covers the ends of bones at a
joint. The degeneration of cartilage is the most common cause of
osteoarthritis –– the pads wear away, leaving bone grinding against
bone.

What the researchers found is that it isn't just
any kind of friction that leads to the irreversible wear and tear on
the material. "It is currently believed that a high-friction force, or
‘coefficient of friction,' is the primary factor in surface wear and
damage," said Israelachvili. "What we found is that this is not the
case."

The critical feature is not a high-friction
force, but what is known as "stick-slip" friction, or, sometimes,
"stiction." Both are characterized by surfaces that initially stick
together, and then accelerate away quickly once the static friction
force is overcome. With stick-slip friction, the surfaces eventually
pull slightly apart and slide across each other, stick again, and pull
apart, causing jerky movements.

"That's when things get damaged microscopically," said Israelachvili.

Stick-slip is a common phenomenon. It is responsible for
everything from computer hard drive crashes and automobile failures, to
squeaking doors and music.

"The same thing happens
with a violin string: Even if you're pulling the bow steadily, it's
moving in hundreds or thousands of little jerks per second, which
determine the sound you hear," Israelachvili said. Each little jerk, no
matter how submicroscopic, is an impact, and over time the accumulation
of these impacts can deform surfaces, causing irreparable damage ––
first microscopically, then growing to macroscopic (large, visible)
dimensions. That's when you have to change the string.

But it's not easy to tell the difference between types of friction at
the microscopic level, where it all begins. Smooth-sliding joints might
feel the same as those undergoing stiction, or the even more harmful
stick-slip, especially in the early stages of arthritis. But, when
measured with an ultra-sensitive and high-resolution instrument like the
SFA, each type of friction revealed its own characteristic profile:
Smooth-sliding joints yielded an almost smooth constant line (friction
force or friction trace); joints with stiction showed up as a peak, as
the "sticking" was being overcome, followed by a relatively smooth line;
while stick-slip shows the jagged saw-tooth profile of two surfaces
repeatedly pulling apart, sticking, and pulling apart again. According
to the scientists, these measurements could be recorded by placing an
acoustic or electric sensing device around joints, giving a signal
similar to an EKG.

"With a well-designed sensor, this
could be a good way to measure and diagnose damage to the cartilage,"
said Banquy. It could be used to measure the progression, or even the
early detection of symptoms related to arthritis.

For a
country that's facing a geriatric population explosion, dealing with
issues associated with old age is no small matter. Early detection of
conditions like arthritis has been a priority for many years, as baby
boomers advance in age.

However, the functioning of
joints is more complicated, said Israelachvili. The scientists will
continue their work by studying synovial fluid –– the lubricating fluid
between two cartilage surfaces in joints –– that also plays a major role
in whether or not the surfaces wear and tear, and the synergistic roles
of the different molecules (proteins, lipids, and polymers) that are
all involved in lubricating and preventing damage to our joints.

"There are a number of directions to take, both fundamental
and practical," Israelachvili said. "But it looks as if we need to focus
our research on finding ways to prevent stick-slip motion, rather than
lowering the friction force."